Biofilm-like properties have been discovered on the sea surface, but the mechanistic understanding how a biofilm develops on the sea surface remains unknown. For this reason, we requested ship time on the RV Poseidon to conduct free-floating mesocosms experiments in the Balearic Sea to monitor the development of biofilm-like properties, i.e. abundance and uniqueness of bacterial community, diversity of phytoplankton, and the accumulation of gel-like particles and dissolved chemical tracers. We have investigated the role of light and UV radiation, and microbes themselves in forming biofilm-like habitats within the SML. With likely unique microbial interactions and photochemical reactions, a biofilm-like habitat at the sea surface may act as a biochemical reactor with implications to ocean and climate science, e.g. marine carbon cycle, diversity and air-sea interaction.